<?php
if(!isset($_SESSION))
    session_start();
?>
<html>
<head> 
<title>乔这一家子论坛(qiao.com)</title> 
</head>
<body>
<?php
if(isset($_SESSION['repeat'])){
  echo "用户名已存在";
  $_SESSION['repeat']=NULL;
}
?>
<form name="input" action="main.php" method="post">
用户名: <input type="text" name="username"><br>
密码: <input type="text" name="password"><br>
验证码: <input type="text" name="checkcode"><br>
<input type="submit" value="提交">
</form>

</body>
</html>
